1. Field of the Invention
The present invention relates to a control system for an automatic transmission.
2. Related Art
The automatic transmission of the prior art is equipped with a torque converter, acting as a fluid coupling for receiving the rotation generated by an engine, and a gear-change mechanism for changing the speed of the rotation transmitted from the torque converter. The gear-change mechanism is equipped with a planetary gear unit composed of a plurality of gear elements for effecting shifting in accordance with a shift pattern which is preset to correspond to the vehicle speed, the throttle opening and so on.
The conventional automatic transmission allows for selection of a (parking) P-range, a (reverse) R-range, a (neutral) N-range, a (drive) D-range, a (second) S-range, a (low) L-range and so on. If the N-range is switched to the D-range by operation of the shift lever, for example, the rotation of the engine in the idling state is transmitted through the torque converter to the gear-change mechanism, to cause the vehicle to advance little by little without any depression of the accelerator pedal, i.e. the so-called "creep phenomenon".
In case, therefore, it is detected that a range such as the D-range, the S-range or the L-range (hereinafter a "forward range") for running the vehicle forward has been selected, that the accelerator pedal has been released, that the brake pedal is depressed and that the vehicle speed is substantially "0", the forward clutch which is applied in forward running of the vehicle, i.e. in a forward range, (hereinafter "first clutch") is brought into slipping engagement to establish a pseudo-neutral state so that the aforementioned creep phenomenon may be prevented. The pseudo-neutral state is intended to reduce vibration and improve fuel economy.
In this pseudo-neutral state (hereinafter "neutral control state"), it is possible, when the accelerator pedal is depressed, to reapply the first clutch, for example, to prevent the delay due to stroke loss of the piston within the servo which operates the first clutch piston, to prevent racing of the engine, and to prevent shock of engagement.
When the first clutch is to be released, moreover, by first abruptly reducing the oil pressure to the hydraulic servo of the first clutch immediately before the first clutch starts its release and subsequently, by gently reducing that oil pressure, the releasing shock is suppressed, and the releasing time period is shortened, while retaining the effects of reducing the vibration and improving the fuel economy to the maximum (as disclosed in Japanese Patent Application Laid-Open No. 79562/1993).
However, in the above-described system, because the oil pressure of the hydraulic servo of the first clutch is always reduced to the same set value, release shock may occur, or the effects of vibration reduction or improvement in the fuel economy may not be retained to the maximum.
If the starting conditions for the neutral control state are satisfied at the time of a fast idling or at the time of driving an accessory such as the air conditioner, for example, the oil pressure of the hydraulic servo of the first clutch is abruptly lowered to the aforementioned set value. However, at the time of the fast idling or at the time of driving an accessory such as the air conditioner, the idling R.P.M., i.e., the engine R.P.M., will rise to augment the input torque. As a result, by always lowering the oil pressure to the same set value, the first clutch may not restore to its state immediately before release, so that the next release is abrupt and accompanied by shock.
FIG. 20 is a first time chart illustrating the control system of an automatic transmission of the prior art, and FIG. 21 is a second time chart illustrating the control system of an automatic transmission of the prior art. In FIGS. 20 and 21 N.sub.E designates engine R.P.M., T.sub.0 designates output torque, P.sub.C1 designates C-1 oil pressure; N.sub.C1 designates the input side R.P.M. of the first clutch; and T.sub.C1 designates the torque capacity of the first clutch. This torque capacity T.sub.C1 changes to correspond to the C-1 oil pressure P.sub.C1.
FIG. 20 illustrates the case in which engine R.P.M. N.sub.E rises by a predetermined amount from the engine R.P.M. N.sub.E ' in an ordinary idling state due to operation of an accessory. In such a case, if the starting conditions for the neutral control state are satisfied at time A, the C-1 oil pressure P.sub.C1 is abruptly lowered. Since, however, the aforementioned set value is set with reference to the ordinary idling state, the input torque T.sub.E exceeds the torque capacity T.sub.C1, and the input side R.P.M. N.sub.C1 of the first clutch undergoes a steeper rise than an ordinary value N.sub.C1 ' so that the first clutch is abruptly released. As a result, the output torque T.sub.0 fluctuates to the extent of causing releasing shock.
If, moreover, the starting conditions for the pseudo-neutral state are satisfied when the engine R.P.M. N.sub.E is lowered at idle by some cause, the oil pressure of the hydraulic servo of the first clutch is abruptly lowered to the aforementioned set value. Since, however, the input torque T.sub.E decreases according to the decrease of the engine R.P.M. N.sub.E, the first clutch cannot be restored to its proper position for initiation of release and, therefore, the releasing time period is shortened so that the effects of vibration reduction and improvement in fuel economy cannot be realized to the maximum.
Likewise, if the accelerator pedal is released at stalling (or vehicle starting) and if the starting conditions for the pseudo-neutral state are satisfied at point A, as illustrated in FIG. 21, the C-1 oil pressure P.sub.C1 is abruptly reduced. However, because the engine R.P.M. N.sub.E rises upon stalling and the input torque T.sub.E increases, the first clutch cannot be restored to its proper position for initiation of release, as in the case of FIG. 20, so that the output torque T.sub.0 seriously fluctuates. After this, the engine R.P.M. N.sub.E is gradually lowered as the accelerator is released, and the input torque T.sub.E accordingly becomes lower than the torque capacity T.sub.C1 so that the first clutch engages at point B and, again, the output torque T.sub.0 fluctuates to the extent of causing releasing shock.